The following relates to the diagnostic imaging arts. It particularly relates to primer sequences for steady state magnetic resonance imaging, and will be described with particular reference thereto. However, the following relates more generally to primer sequences for magnetic resonance imaging of various types.
Steady state magnetic resonance imaging includes a number of substantially similar techniques which are known by various nomenclatures in the art, such as completely balanced steady state free precession imaging (CBASS), fast imaging with steady state precession (FISP and trueFISP), fast imaging employing steady state acquisition (FIESTA), balanced fast field echo (BFFE), and is gradient recalled acquisition of the steady state (GRASS). Steady state imaging techniques provide rapid imaging (TR about 2-5 milliseconds) and contrast that is proportional to a ratio of spin lattice and spin-spin relaxation times (that is, T2/T1) with tip angles close to the Ernst angle. Strong contrast between fluids such as cerebro-spinal fluid and blood relative to other tissues is obtained, making CBASS useful for head and spine imaging, for vascular imaging throughout the body, for cardiac imaging, and the like.
While steady state imaging provides rapid imaging times, as low as 128 milliseconds, disadvantages reside in the slow convergence to steady state, and in the need to continuously pulse the spin system in an uninterrupted fashion in order to properly maintain the steady state magnetization condition.
To prepare the magnetization, the spin system is typically pulsed for about three times the spin-lattice relaxation time (that is, about 3×T1) to allow magnetization to reach steady state. Since spin-lattice T1 relaxation times for fluids such as the cerebro-spinal fluid are about two seconds, signal preparation time far exceeds signal acquisition time. The short TR of steady state imaging sequences also calls for high gradients and is rapid switching, which leads to significant gradient heating during the signal preparation period.
During imaging it is often desirable to pre-saturate particular nuclear signals such as those emanating from fat or flowing blood to improve the diagnostic usefulness of the resultant images. Application of a continuous train of RF pulses during balanced steady state free precession imaging makes inserting the special sections of pulse sequence to perform pre-saturation operations difficult without incurring either a heavy time penalty to restart the steady state or additional image artifacts.
Similarly, the long preparation period employed for preparing the steady state signal substantially limits the use of inversion recovery, T2 contrast, or black blood preparation sequences to modify the contrast available with basic steady state imaging.
To alleviate such problems, priming sequences have been developed which reduce the conditioning time for preparing the nuclear magnetic resonance signal for steady state imaging. However, these priming sequences are complex, non-intuitive, or are not readily adapted to different steady state imaging conditions. For example, complex, non-intuitive priming sequences are not readily adapted for imaging at several different spectral offsets. This limits their application in phase-cycled CBASS, which combines images obtained at two or more spectral offsets to reduce static banding artifacts. These sequences are also not well suited for storing existing steady state magnetization back along the longitudinal axis of the rotating frame, so that pre-saturation operations can be performed periodically during the imaging period, followed by rapid regeneration of the stored steady state signal.
The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.